1. Field of the Invention
The present invention relates to a power amplifier.
2. Background Art
With a power amplifier used in a transmitter-receiver, there is a problem that reception band noise increases with increase of electric power leaking out into a reception band. The amount of electric power leaking out into the reception band is therefore specified.
One of causes of deterioration of reception band noise characteristics is differential frequency noise. Differential frequency noise is noise of the difference between the frequency of a transmission frequency and a reception frequency. When differential frequency noise is mixed in a transmission frequency signal, the electric power of the noise is amplified to cause degradation of a reception band noise characteristic. A power amplifier having a function to reduce the impedance at a differential frequency as a means for coping with such differential frequency noise, as disclosed in, for example, Japanese Patent Laid-Open No. 2008-28635, has been known.
Other prior art includes Japanese Laid-Open Patent Publication No. H10-209769, Japanese Laid-Open Patent Publication No. H9-83268, Japanese Laid-Open Patent Publication No. 2007-174442, and Japanese Laid-Open Patent Publication No. 2005-143079.
There are a plurality of frequency bands determined in accordance with communication standards as reception frequency bands and transmission frequency bands. More specifically, there are reception and transmission frequency bands of about several hundred to a thousand and several hundred megahertz. A differential frequency is the difference between a reception frequency band and a transmission frequency band and is ordinarily about several ten megahertz.
Technique using a filter circuit for removing differential frequency noise in combination with a power amplifier, described in Japanese Patent Laid-Open No. 2008-28635 have been used. In a case where a resonance circuit is used as the filter circuit, the resonance frequency determined by an inductor and a capacitor is set to a value of about several ten megahertz in correspondence with the differential frequency. This resonance frequency is lower by an order of magnitude than the reception band frequency.
Setting the resonance frequency to a value close to the differential frequency requires selecting substantially large capacitance and inductance values which are much larger than the values realized by a metal-insulator-metal (MIM) capacitor and a spiral inductor on a semiconductor substrate. In actuality, it is necessary to use surface mount devices as the inductor and capacitor. The size of the power amplifier is considerably increased due to the existence of such indispensable surface mount devices, and there is a demand for power amplifiers smaller in size.